The present invention relates generally to an optical transmitter, and more specifically relates to an optical transmitter to be applied to a wavelength multiplex (wavelength division multiplex) communication system with the use of a plurality of lightwave signals each having different wavelength.
Wavelength multiplex optical communications where a plurality of light signals each having different wavelength are transmitted in a light transmitting line is important to achieve even higher performance and a lower cost in the communications field and the optical information processing field.
In wavelength multiplex optical communications, it is essential to stabilize and maintain wavelengths of light sources for a plurality of channels to predetermined values. Presently, the wavelengths or frequencies of channels that are to be used in the wavelength multiplex optical communications are fixed to specified values with a 50 or 100 GHz spacing (approximately 0.4 or 0.8 nm spacing) in detail through international standardization by the International Telecommunication Union (ITU). Therefore, it is one of the major problems how to tune the wavelength of a semiconductor laser serving as a light source and stabilize it in conformity to the standard values (ITU grids). Conventionally, in wavelength multiplex optical communication equipment that requires a high degree of reliability, a wavelength monitor and a feed back circuit are provided for each channel to achieve the stabilization of the wavelength of the light source and also a standby light source is provided for each channel in case of failure. Moreover, it is necessary to fabricate distributed feedback (DFB) lasers serving as individual light sources so that their wavelengths locate exactly in predetermined narrow wavelength regions. For this reason, there remains a large problem in the production yield of the semiconductor lasers. These problems are great hindrance against the miniaturization and cost reduction of a transmitter and will become increasingly significant with the decrease of wavelength channel spacing and the increase of the number of the channels.
A light source has been studied that uses a tunable semiconductor laser diode as a light source and is capable of tuning its wavelength over a desired wavelength band covering a plurality of channels. However, in this case, a wavelength stabilization technique for fixing the wavelength of a laser on the ITU grids is required in addition to the realization of a tunable semiconductor laser whose wavelength can be continuously scanned efficiently and easily. To answer this requirement, a tunable/wavelength-stabilized light source has been proposed which incorporates a wavelength tuning mechanism based on the thermo-electric cooler (Peltier) and a thin film etalon filter, which has, however, a problem that its construction and production are complicated as well as a problem of large power consumption. By the way, for this kind of a light source for the wavelength multiplex communication, a paper in IEEE Photonics Technology Letters, Vol.4, p.321 (1992) can be given as an example.
Presently, in optical communication systems for long-distance trunk lines, signal transmission speeds of 2.5 to 10 Gb/s are principally in use, and hence an external modulation method is essential. Therefore, a modulator-integrated light source is preferable that is formed by monolithically integrating a semiconductor laser and an external light modulator.
Therefore, it is a main object of the present invention to provide a light transmitter for wavelength multiplex optical communications that is easy to fabricate and capable of tuning its wavelength to the above-mentioned standard channels. Specifically, an object is to provide an optical transmitter suitable for long-distance optical communication with the use of a semiconductor laser with the external modulator monolithically integrated.
To achieve the above-mentioned object, an optical transmitter according to the present invention can be constructed to comprise: a semiconductor laser for emitting light having a single mode wavelength, a control circuit for tuning the above-mentioned wavelength in a wide range of wavelengths and stabilizing it to a specific wavelength, and optical means for performing optical/electrical conversion of the above-mentioned laser light to a current and providing it to the above-mentioned control circuit.
In a preferred embodiment of the present invention, the above-mentioned semiconductor laser and the above-mentioned optical means are monolithically integrated or hybrid integrated on a substrate. Moreover, the above-mentioned semiconductor laser is constructed in a heater-integrated tunable structure wherein a thin film heater is mounted just above an upper electrode of the distributed feedback (DFB) semiconductor laser or a distributed reflection (DBR) semiconductor laser which have excellent wavelength tunability. Moreover, the above-mentioned optical means is composed of an optical device having a plurality of single-peaked frequency characteristics at a plurality of discrete frequencies, the peaks being similar to one another, and the above-mentioned control circuit comprises a wavelength control circuit for tuning the above-mentioned single mode wavelength and an injection current control circuit for controlling the injection current of the above-mentioned semiconductor laser and constructed so that a difference and a summation of the signal components of adjacent frequencies of the above-mentioned discrete frequencies are fed to input sections of the above-mentioned wavelength control circuit and the above-mentioned injection current control circuit, respectively.